The field of the invention nanoporous polymers.
Decreasing size and increasing density of functional elements in integrated circuits has generated a continuing demand for insulating materials with reduced dielectric constants. Among other approaches, inclusion of air into an insulating material has been successfully used to reduce the dielectric constant of the material, and various methods of introducing air into materials are known in the art.
In one method, void carriers such as hollow glass micro-spheres are incorporated into a polymeric matrix. For example, Yokouchi et al. teach in U.S. Pat. No. 5,593,526 a process for producing a wiring board in which hollow or porous glass spheres are covered with a ceramic coating layer, and wherein the coated glass spheres are then mixed with a glass matrix. Yokouchi""s glass spheres help to reduce the dielectric constant of the wiring board, however, require coating by relatively cumbersome and expensive methods such as chemical vapor deposition, etc. Moreover, in order to create a stable structure between the glass matrix and the coated spheres, the mixture has to be fired at temperatures of about 1000xc2x0 C., which is unacceptable for most, if not all integrated circuits.
In order to circumvent at least some of the problems associated with high-temperature curing, Sato et al. describe in U.S. Pat. No. 5,194,459 an insulating material that is formed from a network of hollow gas filled microspheres entrapped in a cured crosslinked fluorinated polymer network. Sato""s materials dramatically reduce the temperature requirements as compared to Yokouchi""s materials. Furthermore, Sato""s materials can be coated onto appropriate materials in a relatively thin layer while retaining tensile strength. However, all of Sato""s polymers include fluorine, which tends to reduce adhesion of the polymer to the materials employed in the fabrication of integrated circuits. Moreover, fluorine is known to cause corrosion of metal conductor lines. Still further, since the glass spheres in Sato""s polymer network are not covalently bound to the surrounding network, the mechanical integrity of the porous polymer composition may be less than desirable under certain conditions.
In another method, a thermolabile component is incorporated into a polymeric material, and after curing the polymeric material, the thermolabile component is destroyed by heating. For example, Hedrick et al. describe in U.S. Pat. No. 5,776,990 blending of a thermostable polymer with a thermolabile (thermally decomposable) polymer. The blended mixture is subsequently crosslinked and the thermolabile portion thermolyzed. Blending a thermostable and a thermolabile polymer is conceptually simple, and allows relatively good control over the amount of porosity in the final polymer. However, positional control of the voids is generally difficult to achieve, and additional problems may arise where control over homogeneity and size of the voids is desirable.
Alternatively, the thermolabile portion can be grafted onto the polymeric strands. For example, block copolymers may be synthesized with alternating thermolabile blocks and thermostable blocks. The block copolymer is then heated to thermolyze the thermolabile blocks. In another approach, thermostable blocks and thermostable blocks carrying thermolabile portions can be mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. While incorporation of a thermolabile portion generally improves control over pore size and distribution, the synthesis of such polymers is frequently challenging.
Regardless the approach of incorporation of the thermolabile compound into the polymeric network, various difficulties still persist. Most disadvantageously, almost all polymer systems exhibit only a relatively narrow window between the curing (i.e., crosslinking) temperature of the polymeric strands and the temperature at which the thermolabile compound disintegrates. Consequently, the thermolabile compound often begins to degrade when the polymeric network is not yet sufficiently crosslinked, typically leading to pore collapse and unsatisfactory reduction in structural stability and dielectric constant.
Although various methods of generating nanoporosity are known in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide improved methods and compositions to generate nanoporous materials.
The present invention is directed to compositions and methods of forming nanoporous materials. In one aspect of the inventive subject matter, the composition comprises a polymeric network that includes a porogen and a photoinitiator disposed within the polymeric network. The photoinitiator produces a reactive species upon irradiation, and the reactive species reacts with the porogen in a degradation reaction that degrades at least some of the porogen.
In a preferred aspect of the inventive subject matter, the polymeric network comprises a crosslinked polymer, and it is even more preferred that the network comprises a crosslinked poly(arylene ether). Contemplated porogens comprise organic compounds, preferably oligo- or polymeric compounds, which may advantageously include acid hydrolysable groups, such as trimethylsilyl groups, t-butylcarboxy groups, or ketal groups.
In a further aspect of the inventive subject matter, the photoinitiator comprises a salt of an acid, preferably a triarylsulfonium salt of an acid, and the photoinitiator generates upon irradiation an acid as the reactive species.
In a still further aspect of the inventive subject matter, the degradation reaction comprises unmasking of a protecting group, a de-crosslinking reaction, or a fragmentation/depolymerization of a polymer.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawing.